Refactor out map.el usage

After some profiling, it turns out map-put and map-delete are 5-7x
slower (more on Emacs 25) than delq, setf/alist-get and add-to-list for
small lists (under 250 items), which is exactly how I've been using
them.

The only caveat is alist-get's signature is different on Emacs 25, thus
a polyfill is necessary in core-lib.

modules/feature/lookup/autoload/devdocs.el

@@ -9,7 +9,7 @@ DOCSET (a string).
 See `devdocs-alist' for the defaults. "
   (after! (:when (boundp 'devdocs-alist))
     (dolist (mode (doom-enlist modes))
-      (map-put devdocs-alist mode docset))))
+      (setf (alist-get mode devdocs-alist) docset))))
 
 ;;;###autoload
 (def-setting! :devdocs (modes docset)

modules/feature/lookup/autoload/lookup.el

@@ -72,7 +72,7 @@ properties:
                       "Search on: "
                       (mapcar #'car +lookup-provider-url-alist)
                       nil t)))
-          (map-put +lookup--last-provider key provider)
+          (setf (alist-get +lookup--last-provider key) provider)
           provider))))
 
 (defun +lookup--symbol-or-region (&optional initial)
@@ -318,7 +318,9 @@ for the provider."
           (user-error "The search query is empty"))
         (funcall +lookup-open-url-fn (format url (url-encode-url search))))
     (error
-     (map-delete +lookup--last-provider major-mode)
+     (setq +lookup--last-provider
+           (delq (assq major-mode +lookup--last-provider)
+                 +lookup--last-provider))
      (signal (car e) (cdr e)))))
 
 ;;;###autoload